Push bar exit devices are mounted on the egress side of the door. They can be mounted for rim, mortise, surface vertical rods, or concealed vertical rods applications. Push bar exit devices feature an enclosed mechanism case with a push bar area to allow egress. Some electrified latch pullback mechanisms for exit devices allows for remote keyless access control where exit devices are used or required. Common applications include conditions such as hospitals, airports, schools, churches and they are commonly specified in applications that require automatic door openers. The device may be continuously energized allowing for a push/pull condition if desired.
Electrified latch pullback modifications are a common product within the industry. They are typically solenoid or motor driven devices. Motor driven devices are usually coupled to an acme style threaded leadscrew combination to translate the rotational output force of the motor to the linear force most commonly required to actuate the latch of an exit device. Because of the high contact area required in this type of leadscrew, the friction losses are generally very high. Because of these losses, larger motors and/or finer thread pitches are required to create the linear force requirements for the specific application.
However, larger motors and/or finer pitches cannot always satisfy the requirements for each exit device. Size limitations may prevent larger motors from fitting within the enclosed mechanism case. Finer thread pitches create several other problems. First, the finer pitch often results in a slower actuation time. More problematic however, is the requirement for the springs within the device to return the leadscrew to the starting point for “fail-safe” operation.
Fail-safe operation within an exit device requires that a non-energized device return to a locked or latched state. Power is generally applied to the device to unlock the latch. Because of the fail-safe requirement, driving the motor in reverse to lock the latch would not be possible. Thus springs must be used to store mechanical energy during latch retraction, then when released, to force the lead screw back to the starting position once power is removed. Adding these springs to a device then requires more power output from the motor, requiring a larger motor. Motors, and more particularly, stepper motors, have an inherent magnetic and mechanical friction observed when rotating the motor shaft in a non-powered state. This friction force increases as the motor size increases. All of these required forces must be balanced for a system to work correctly.
The drive forces of the acme, or a similar style lead screw, is further complicated by the fact that the friction loss characteristics are different when the screw is “back-driven”. Normally, lead screws are used to convert rotary motion into linear motion. Back driving is the result of the load pushing axially on the screw or nut to create rotary motion, this in turn, rotating the motor. As a thread pitch is made finer, the friction losses increase, nearly exponentially, to the point that the screw can no longer be back driven. This is known as self-locking. As this relates to the previously mentioned springs used for fail-safe return, only a small range of very coarse thread lead screws can be used. Courser thread pitches require more power from a larger motor. All of these constraints often lead to an impossibility of driving an exit device with a motor and lead screw within the existing specified enclosure constraints.